Ale group. doi:10.1371/journal.pone.0053847.tmanding challenge for the city. The first incidence of 2009 H1N1 in Shenzhen was reported on 28 May 2009, and the peak of the pandemic occurred in September that year [12].Study Subjects ISerum sampling. In this cross-sectional serological study, the study subjects were individuals with or Docosahexaenoyl ethanolamide manufacturer without presence of influenza-like illness (ILI) who went to medical visit in hospitals in 7 districts of Shenzhen. They were recruited by stratified random sampling according to age groups: ,5 years, 6?5 years, 16?5 years, 26?9 years, and above 60 years. In total 1,427 serum samples were collected from individuals aged from 0 to 85 during 2009, of which 535 were recruited in March (before the H1N1 pandemic) and 892 in September 2009 (during the H1N1 pandemic). On average, there were 48.6 males and 58.4 females in March, and 90.6 males and 87.8 females in September in each age group. The detailed information of each age group was listed in Table S1 and Table S2. The questionnaire included age, gender, history of respiratory tract infection, and history of vaccination and the presence or absence of ILI. Based on the questionnaires, no participants recruited in this study had received vaccination against seasonal influenza during the period of 2006?008. Informed consent from each study subject was collected in person or by the guardians. This study was approved by the Institutional Review Board and the Human Research Ethics Committee of the Shenzhen Center for DiseaseControl and Prevention (Shenzhen CDC). Written consent was obtained from the participants or the guardians of children. Hemagglutination inhibition test. The human serum samples were treated with a receptor-destroying enzyme (Denka Seiken Co., Ltd, Tokyo, Japan) in a ratio of 4:1 (volume: volume) at 37uC overnight to eliminate non-specific inhibitors of hemagglutination. Then the samples were tested for HA-specific antibodies by a standard hemagglutination-inhibition (HI) assay [13]. Two seasonal influenza 
A viruses (H1N1 and H3N2) and two seasonal influenza B viruses (B/Y and B/V) were used as antigens to measure the antibodies against each subtype of flu virus in the sera of cohorts. The tested seasonal strains were: A/Tianjin Jinnan/15/2009 (H1N1), A/Fujian Tongan/196/2009 (H3N2), B/Jiangxi Xiushui/32/2009 (Victoria), and B/Guangdong Xinxing/134/2009 (Yamagata). Serum-only controls for each human serum sample without added viral antigen were also assayed in parallel with the virus-specific assays. Only virus-specific assays with titer values greater than or equal to the corresponding serumonly control values were considered. An HI antibody titer of 1:40 or more was considered seropositive. To calculate geometric mean titers (GMTs) for individual cohorts, titers below the lower limit (1:10) were determined at the value of 1:5 [14,15]. The antibody titers used to calculate GMTs can be found in Supplementary Tables (Table S3, S4, S5, S6, S7, S8, S9, S10, S11, S12, S13, S14, S15, S16, S17, S18).Study Subjects IIIn order to plot the overall trend of ILI MedChemExpress GNF-7 incidences and influenza subtypes in 2009, we used the monthly data of ILI incidences and influenza subtypes tests provided by Shenzhen CDC. The test details are as follows:Clinical nasopharyngeal swab specimens, virus culture and genotyping. Nasopharyngeal swab specimens were col-Table 2. Comparison of Seasonal Influenza Antibody Change before and during the 2009 H1N1 Pandemic for Male (Mean titer.Ale group. doi:10.1371/journal.pone.0053847.tmanding challenge for the city. The first incidence of 2009 H1N1 in Shenzhen was reported on 28 May 2009, and the peak of the pandemic occurred in September that year [12].Study Subjects ISerum sampling. In this cross-sectional serological study, the study subjects were individuals with or without presence of influenza-like illness (ILI) who went to medical visit in hospitals in 7 districts of Shenzhen. They were recruited by stratified random sampling according to age groups: ,5 years, 6?5 years, 16?5 years, 26?9 years, and above 60 years. In total 1,427 serum samples were collected from individuals aged from 0 to 85 during 2009, of which 535 were recruited in March (before the H1N1 pandemic) and 892 in September 2009 (during the H1N1 pandemic). On average, there were 48.6 males and 58.4 females in March, and 90.6 males and 87.8 females in September in each age group. The detailed information of each age group was listed in Table S1 and Table S2. The questionnaire included age, gender, history of respiratory tract infection, and history of vaccination and the presence or absence of ILI. Based on the questionnaires, no participants recruited in this study had received vaccination against seasonal influenza during the period of 2006?008. Informed consent from each study subject was collected in person or by the guardians. This study was approved by the Institutional Review Board and the Human Research Ethics Committee of the Shenzhen Center for DiseaseControl and Prevention (Shenzhen CDC). Written consent was obtained from the participants or the guardians of children. Hemagglutination inhibition test. The human serum samples were treated with a receptor-destroying enzyme (Denka Seiken Co., Ltd, Tokyo, Japan) in a ratio of 4:1 (volume: volume) at 37uC overnight to eliminate non-specific inhibitors of hemagglutination. Then the samples were tested for HA-specific antibodies by a standard hemagglutination-inhibition (HI) assay [13]. Two seasonal influenza A viruses (H1N1 and H3N2) and two seasonal influenza B viruses (B/Y and B/V) were used as antigens to measure the antibodies against each subtype of flu virus in the sera of cohorts. The tested seasonal strains were: A/Tianjin Jinnan/15/2009 (H1N1), A/Fujian Tongan/196/2009 (H3N2), B/Jiangxi Xiushui/32/2009 (Victoria), and 
B/Guangdong Xinxing/134/2009 (Yamagata). Serum-only controls for each human serum sample without added viral antigen were also assayed in parallel with the virus-specific assays. Only virus-specific assays with titer values greater than or equal to the corresponding serumonly control values were considered. An HI antibody titer of 1:40 or more was considered seropositive. To calculate geometric mean titers (GMTs) for individual cohorts, titers below the lower limit (1:10) were determined at the value of 1:5 [14,15]. The antibody titers used to calculate GMTs can be found in Supplementary Tables (Table S3, S4, S5, S6, S7, S8, S9, S10, S11, S12, S13, S14, S15, S16, S17, S18).Study Subjects IIIn order to plot the overall trend of ILI incidences and influenza subtypes in 2009, we used the monthly data of ILI incidences and influenza subtypes tests provided by Shenzhen CDC. The test details are as follows:Clinical nasopharyngeal swab specimens, virus culture and genotyping. Nasopharyngeal swab specimens were col-Table 2. Comparison of Seasonal Influenza Antibody Change before and during the 2009 H1N1 Pandemic for Male (Mean titer.
